1. Camera2基础预览

任务说明

本案例将使用Camera2 API实现相机实时预览，将相机画面显示在SurfaceView上。你需要完成以下核心工作：获取相机权限、打开相机、创建预览会话、持续输出预览帧到SurfaceView。

实现方案

整体流程为：CameraManager.openCamera()打开相机 → 通过CameraDevice.createCaptureSession()创建预览会话 → 使用CaptureRequest.Builder构建预览请求并设置Surface为输出目标 → 循环发送预览请求。关键点在于正确处理相机权限请求、选择合适的后置相机ID，以及管理相机设备的生命周期。

代码实现

// AndroidManifest.xml 中添加权限
<uses-permission android:name="android.permission.CAMERA" />
<uses-feature android:name="android.hardware.camera2" android:required="true" />

// activity_camera_preview.xml
<SurfaceView
    android:id="@+id/surface_view"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

// CameraPreviewActivity.java
public class CameraPreviewActivity extends AppCompatActivity {
    private SurfaceView surfaceView;
    private SurfaceHolder surfaceHolder;
    private CameraManager cameraManager;
    private String cameraId;
    private CameraDevice cameraDevice;
    private CameraCaptureSession captureSession;
    private CaptureRequest.Builder previewRequestBuilder;

    private final CameraDevice.StateCallback stateCallback = new CameraDevice.StateCallback() {
        @Override
        public void onOpened(@NonNull CameraDevice camera) {
            cameraDevice = camera;
            createPreviewSession();
        }

        @Override
        public void onDisconnected(@NonNull CameraDevice camera) {
            cameraDevice.close();
            cameraDevice = null;
        }

        @Override
        public void onError(@NonNull CameraDevice camera, int error) {
            cameraDevice.close();
            cameraDevice = null;
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_camera_preview);

        surfaceView = findViewById(R.id.surface_view);
        surfaceHolder = surfaceView.getHolder();
        surfaceHolder.addCallback(new SurfaceHolder.Callback() {
            @Override
            public void surfaceCreated(SurfaceHolder holder) {
                if (checkCameraPermission()) {
                    openCamera();
                } else {
                    requestCameraPermission();
                }
            }

            @Override
            public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {}

            @Override
            public void surfaceDestroyed(SurfaceHolder holder) {
                closeCamera();
            }
        });

        cameraManager = (CameraManager) getSystemService(Context.CAMERA_SERVICE);
    }

    private boolean checkCameraPermission() {
        return ContextCompat.checkSelfPermission(this, Manifest.permission.CAMERA)
                == PackageManager.PERMISSION_GRANTED;
    }

    private void requestCameraPermission() {
        ActivityCompat.requestPermissions(this,
                new String[]{Manifest.permission.CAMERA}, 100);
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions,
                                           @NonNull int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        if (requestCode == 100 && grantResults.length > 0
                && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
            openCamera();
        }
    }

    private void openCamera() {
        try {
            // 选择后置摄像头
            for (String id : cameraManager.getCameraIdList()) {
                CameraCharacteristics characteristics =
                        cameraManager.getCameraCharacteristics(id);
                Integer facing = characteristics.get(CameraCharacteristics.LENS_FACING);
                if (facing != null && facing == CameraCharacteristics.LENS_FACING_BACK) {
                    cameraId = id;
                    break;
                }
            }
            if (cameraId == null) {
                cameraId = cameraManager.getCameraIdList()[0];
            }
            cameraManager.openCamera(cameraId, stateCallback, null);
        } catch (CameraAccessException | SecurityException e) {
            e.printStackTrace();
        }
    }

    private void createPreviewSession() {
        try {
            Surface surface = surfaceHolder.getSurface();
            previewRequestBuilder =
                    cameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);
            previewRequestBuilder.addTarget(surface);

            cameraDevice.createCaptureSession(
                    Collections.singletonList(surface),
                    new CameraCaptureSession.StateCallback() {
                        @Override
                        public void onConfigured(@NonNull CameraCaptureSession session) {
                            if (cameraDevice == null) return;
                            captureSession = session;
                            previewRequestBuilder.set(
                                    CaptureRequest.CONTROL_MODE,
                                    CameraMetadata.CONTROL_MODE_AUTO);
                            try {
                                captureSession.setRepeatingRequest(
                                        previewRequestBuilder.build(), null, null);
                            } catch (CameraAccessException e) {
                                e.printStackTrace();
                            }
                        }

                        @Override
                        public void onConfigureFailed(@NonNull CameraCaptureSession session) {
                            // 会话配置失败，可在此提示用户
                        }
                    }, null);
        } catch (CameraAccessException e) {
            e.printStackTrace();
        }
    }

    private void closeCamera() {
        if (captureSession != null) {
            captureSession.close();
            captureSession = null;
        }
        if (cameraDevice != null) {
            cameraDevice.close();
            cameraDevice = null;
        }
    }

    @Override
    protected void onPause() {
        super.onPause();
        closeCamera();
    }
}

落地与验证

运行方式：将上述代码分别放入对应文件，在Android Studio中构建并部署到已授予相机权限的真机或模拟器（API 21+）。

验证方法：启动应用后，SurfaceView区域应实时显示后置相机拍摄的画面。旋转设备时画面方向会随传感器方向变化（本案例未做方向校正，属正常现象）。

判定标准：画面流畅无黑屏，切换后台再返回时预览能自动恢复（通过onPause关闭相机、surfaceCreated重新打开实现）。若首次启动弹出权限对话框，点击“允许”后预览正常显示，即视为通过。